In an MRI apparatus, various pulse sequences are used corresponding to the imaging target and image type. Among those pulse sequences, a pulse sequence referred to as the fast spin echo method (FSE method) is widely used due to the short imaging time and the diversity of image types that can be imaged.
In the FSE method, using a plurality of 180-degree RF pulses, a plurality of spin echoes are obtained by repeatedly inverting an echo generated after one spin excitation by a 90-degree RF pulse. Highly accurate control is required for an RF pulse to generate these echoes and a gradient magnetic field pulse that encodes echoes. In a pulse sequence that is software to achieve the FSE method, an intensity and a timing of an ideal RF pulse and a gradient magnetic field pulse are described. However, since errors actually occur for a gradient magnetic field pulse and an RF pulse generated by hardware, it is difficult to achieve a theoretical pulse sequence in a static magnetic field space. Therefore, each echo will include a magnetic field error, and artifacts occur in an image that uses such an echo.
Various improvement measures have been proposed for this problem. For example, in the Non-patent Literature 1 and Patent Literature 1, a method is proposed that collects raw data itself in a state close to an ideal as possible by obtaining reference data by pre-scan to adjust a pulse sequence using the results. Specifically, this method measures a first echo and a second echo by pre-scan that uses a modified FSE pulse sequence where a phase encoding is not used in the first step, calculates a phase error between these echoes, and adjusts a 180-degree RF pulse so that the phase error is minimized. Next, as the second step, this method calculates a difference between gradients of phase profiles of a first echo and a second echo measured by pre-scan similarly and corrects a read gradient magnetic field so that the phase error is minimized.